An antenna can include one or more structural electrical elements each providing a bi-directional transition between a guided electrical wave and a free-space propagating wave. A resonant frequency of an antenna can be related to the electrical length of the antenna. Often, an antenna is tuned for a specific resonant frequency and may be effective for a range of frequencies usually centered around the resonant frequency. Other properties of antennas, such as radiation pattern and impedance, change with frequency.
Typically, an antenna is designed for efficient operation over a certain band of frequencies. The antenna size is related to the wavelength of radiation that the antenna is supposed to receive or transmit. An efficient dipole antenna can be constructed with a size of λ/2, where λ represents a wavelength corresponding to the resonant frequency of the antenna. A monopole type of antenna at λ/4 length is efficient if mounted on an adequately large ground plane or if supplied with radials, which can be wires or other conductors disposed perpendicular to the monopole (e.g., on or in the ground). The λ/4 antennas are the most prevalent type used in handheld devices such as mobile communication devices, e.g., cell phones. Full λ antennas are usually not practical since they are too long at the frequencies of interest. For example, the length of a 30 MHz one λ antenna is 10 meters, which is too large for most mobile platforms.
Communication antennas, including those for vehicles, are generally adapted to receive and/or transmit signals in a particular frequency range. The antennas are sized and configured in order to optimize efficiency at particular frequency ranges. Further, the challenge to miniaturize electronic components also applies to antenna design where the antenna's physical dimensions are strongly linked to the component's performance. As the physical size of communication devices shrink, manufacturers are compelled to shrink the size of the antenna systems as well.